Enhanced removal of Cr (VI) in constructed wetland with Fe-Ni-LDH-modified crayfish shell biochar: Performance, removal pathways and synergy mechanisms of substrate-microorganism.

In this study, Fe-Ni-layered double hydroxide modified crayfish shell biochar substrate (Fe-Ni-LDH@CSBC) was successfully prepared and introduced into constructed wetland (CW) to research the Cr(VI) removal mechanism through substrate adsorption and microbial action. Adsorption experiments demonstrated the equilibrium adsorption capacities of Fe-Ni-LDH@CSBC for Cr(VI) could reach 1058.48 (C=10 mg/L) and 1394.59 mg/kg (C=20 mg/L), respectively. The CW with Fe-Ni-LDH@CSBC showed higher removal efficiencies of COD, TN, TP, and Cr(VI), which respectively reached 72.9 %, 66.8 %, 79.6 %, and 99.3 % with influent Cr(VI) concentration of 50 mg/L. The Fe-Ni-LDH@CSBC stimulated the secretion of extracellular polymeric substances (EPS) (570.6 mg/g VSS), conducive to absorbing Cr(VI) and also delaying or preventing Cr(VI) from entering microorganisms. The increased reduction of Cr(VI) by Fe-Ni-LDH@CSBC was due to its role in enhancing extracellular electron transfer (EET) through boosting cytochrome c, ETSA and NADH levels. Metagenomic sequencing showed that Fe-Ni-LDH@CSBC increased the abundances of resistant microorganisms (Mycobacterium, Mycolicibacterium, and Hyphomicrobium) in CW, and the relative abundance of key functional genes from Cr(VI) reductase (chrR, GST, czcD, zitB, and merR), electron transfer genes (mtrC, fbpC, insB, and afuC) and transport proteins genes (rhtC, pcoB, bpeT, pecM, chrA, and exbB). This research presented an effective method to enhance the removal of Cr(VI) from electroplating wastewater using CW with Fe-Ni-LDH@CSBC.